The operation of an internal combustion engine, such as, for example, a diesel, gasoline, or natural gas engine, may cause the generation of undesirable emissions. These emissions, which may include particulates and oxides of nitrogen (NOx), are generated when fuel is combusted in a combustion chamber of the engine. An exhaust stroke of an engine piston forces exhaust gas, which may include these emissions, from the engine. If no emission reduction measures are in place, these undesirable emissions will eventually be exhausted to the environment.
Research is currently being directed towards decreasing the amount of undesirable emissions that are exhausted to the environment during the operation of an engine. It is expected that improved engine design and improved control over engine operation may lead to a reduction in the generation of undesirable emissions. Many different approaches, such as, for example, exhaust gas recirculation, water injection, fuel injection timing, and fuel formulations, have been found to reduce the amount of emissions generated during the operation of an engine. Aftertreatments, such as, for example, traps and catalysts have been found to effectively remove emissions from an exhaust flow. Unfortunately, the implementation of these emission reduction approaches typically results in a decrease in the overall efficiency of the engine.
Additional efforts are being focused on improving engine efficiency to compensate for the efficiency loss due to the emission reduction systems. One such approach to improving the engine efficiency involves adjusting the actuation timing of the engine valves. For example, the actuation timing of the intake and exhaust valves may be modified to implement a variation on the typical diesel or Otto cycle known as the Miller cycle. In a “late intake” type Miller cycle, the intake valves of the engine are held open during a portion of the compression stroke of the piston. This may result in an improvement in the overall efficiency of the engine.
An engine may be equipped with a variable valve actuation system that provides for selective adjustment of the actuation timing of the engine valves. The variable valve actuation system may be controlled to selectively override the valve actuation timing provided by a conventional cam-driven valve actuation system. In a conventional cam-driven valve actuation system, the engine valves are actuated by a cam arrangement that is operatively connected to the engine crankshaft. A rotation of the crankshaft results in a corresponding rotation of a cam that drives one or more cam followers. The movement of the cam followers results in the actuation of the engine valves. Thus, the shape of the cam governs the timing and duration of the valve actuation.
As described in U.S. Pat. No. 6,237,551 to Macor et al., issued on May 29, 2001, a hydraulically powered valve actuator may be incorporated with a conventional cam assembly to allow for selective implementation of a variation on conventional valve actuation timing. In particular, a variable valve actuator may be disposed between the cam arrangement and the engine valve. The variable valve actuator may include a chamber in which fluid may be sealed to establish a hydraulic link between the cam and the engine valve. When the hydraulic link is established, all of the valve motion provided by the shape of the cam is used to actuate the engine valve. To vary the actuation timing of the engine valve, a control valve may be opened to allow fluid to flow from the chamber. The release of the fluid breaks the hydraulic link between the cam and the engine valve and the engine valve is allowed to close, independently of the shape of the cam. In this manner, a variable valve actuator may be used to selectively vary the actuation timing of an engine valve.
Changes in the properties of the fluid used to operate a variable valve actuation system may change the operation of the actuation system. An unexpected change in a fluid property may change the rate at which fluid flows into and out of the chamber of the hydraulic actuator. An increase or decrease in the fluid flow rate may result in an increase or decrease in the time required for the valve actuator to operate.
For example, when the engine is starting, the operating fluid may have a cold temperature and, thus, a high viscosity. The high viscosity of the fluid increases the amount of time required for the valve actuator to operate. This increased operation time may unexpectedly change the valve actuation timing and reduce or eliminate any performance gains that may have been achieved by implementing a variation on the conventional valve actuation timing.
The control system and method of the present disclosure solves one or more of the problems set forth above.